Nine things to know about sharks

Before you dive into the shark-infested airwaves or start planning that Jaws movie marathon, here are nine things to know about these little known marine predators.

Alice Coyle acoyle@wickedlocal.com @accoyleWL

Shark Week, the Discovery Channel’s annual homage to great whites, hammerheads and other shark species, begins Sunday, July 23. Now in its 30th year, Shark Week started out as programming to educate viewers about sharks and has evolved into a Megladon-sized summer television hit for the cable network. Before you dive into the shark-infested airwaves or start planning that Jaws movie marathon, here are nine things to know about these little known marine predators.

1. Sharks come in many shapes and sizes. In fact there are more than 400 shark species — ranging in size from a few inches to upwards of 52 feet long — swimming around in our oceans. (Rivers, too – see #5)

2. Sharks see things in black and white. With only one type of cone photoreceptor (compared to three for humans) sharks are colorblind. So it’s not the red color of your blood in the water that is attracting sharks, but watch out for their “sixth sense.”

3. Sharks may not be able to see colors, but a sixth sense allows them to sense even the slightest pulses, like that of an electrical field, a muscle movement or a heartbeat. Tiny organs in their snouts enable them to detect these pulses. (It remains unclear if they also “see dead people.”)

4. If you’re not a fan of heavy metal, you’ll want to keep shark meat off the menu. Mercury, arsenic and lead are found in high amounts in shark’s tissue, mostly as a result of their diet. Shark meat has higher levels of heavy metal concentrations than any other fish in the sea.

5. The ocean is not their only turf. Sharks have been known to swim up stream into rivers all around the world. Bull sharks reportedly travel more than 100 miles up Nicaragua’s Rio San Juan and into Lake Nicaragua, and others have made their way more than 2,600 miles up the Amazon River. In the U.S., a pair of fishermen in 1937 reeled in a bull shark in Alton, Illinois that had traveled some 1,160 miles up the Mississippi River. And recently, a harbormaster on Massachusetts’ South Shore said it’s possible sharks could follow their seal prey into the North River. (Read more in this SPECIAL REPORT: Sharks on the South Shore).

6. Sharks use their heads and tails when hunting prey. The hammerhead shark’s cranium gives it twice the edge as a predator. It looks scary and is also used to bludgeon and pin down stingrays while feeding on them. Thresher sharks use their long tails to whip and whack their prey into submission at lightning speed, crippling or cutting the fish into easy-to-eat pieces.

7. Most shark attacks in the U.S. take place in Florida – specifically Volusia County, which includes Daytona Beach in the east-central part of the state. Blacktip and spinner sharks, plentiful in shallow water where people swim, surf and fish, are the species most likely to launch unprovoked attacks – not great whites or bull sharks.

8. More seals have brought more sharks to New England waters. The booming seal population has been attributed to conservation efforts and the 1972 Marine Mammal Protection Act and in recent years reported sightings of great white sharks have increased greatly. Just last month, a family fishing off of Minot’s Ledge Light in Scituate, Mass. reported spotting a great white from their boat; and there have been other shark sightings off Cape Cod and in Plymouth, Mass. While shark sightings have increased, the threat to humans is not necessarily any greater in New England, which has not seen a shark-related death in 81 years.

9. Sharks are both predator and prey. Orcas (killer whales) are known to attack and eat white sharks, especially in the waters off San Francisco. Farther north off British Columbia, packs of orcas gang up and tear apart Pacific sleeper sharks, one of the world’s largest sharks at more than 20 feet long. Smaller predators, including a variety of parasites, also attack sharks. But humans, by far, pose the greatest threat to sharks. While in a typical year sharks kill about six people worldwide, humans kill an estimated 100 million sharks annually, most of them for their fins, meat and oil.

Tune into Shark Week starting Sunday, July 23 at 8 p.m. on the Discovery Channel.